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Cosmic inflation is dead, long live cosmic inflation!

By Michael Slezak

INFLATION is dead, long live inflation! The very results hailed this year as demonstrating a consequence of inflationary models of the universe – and therefore pointing to the existence of a multiverse – may now do the exact opposite. If the results can be trusted at all, they seemingly suggest inflation is wrong, and raise the possibility of universes that predate the big bang.

In March, the team behind the BICEP2 telescope in Antarctica (pictured) announced that they had seen evidence of primordial gravitational waves. These waves were revealed as telltale twists and turns in the polarisation of the cosmic microwave background radiation (CMB), the remnants of the universe’s earliest light.

Physicists hailed the discovery as preliminary confirmation of inflation, the idea that for a sliver of a moment after the big bang, the universe expanded at blistering speed. The theory, the most widely held cosmological idea about the growth of our universe after the big bang, accounts for a number of mysteries, including why the universe is surprisingly flat and smoothly distributed.

Very quickly, though, the BICEP2 finding became shrouded in doubt, as it was revealed that the polarisation pattern could have been caused by cosmic dust. Preliminary results released last week from the space-based Planck telescope suggest that dust could indeed account for the pattern BICEP2 detected.

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But this week, a team of theorists decided to ask&colon; assuming the signal isn’t caused by dust, what exactly does it say about inflation? David Parkinson at the University of Queensland in Australia and his team examined the nature of the apparent gravitational waves, rather than their mere existence, to see if they were the type of waves inflation predicts. And they weren’t.

Most inflationary models require that, as you look at larger and larger scales of the universe, you should see stronger and stronger gravitational waves. In BICEP2’s data, they get weaker. Contrary to what the BICEP2 collaboration said initially, Parkinson’s analysis suggests that the BICEP2 results, if legitimate, actually rule out any reasonable form of inflationary theory.

The BICEP2 results may actually rule out any reasonable form of inflationary theory

Not everyone is giving up so easily. Alan Guth, a physicist at the Massachusetts Institute of Technology who pioneered the concept of inflation, says the analysis is convincing, but not so convincing that he’s ready to abandon the possibility that BICEP2’s data holds a signal in support of more obscure models of inflation. Even if the signal ends up being mainly due to dust, that is not strong evidence against inflation, Guth claims, since many inflationary models predict a much smaller signal that would require more work to find. “If BICEP2 has not seen [evidence of] gravitational waves, then only certain inflationary models are ruled out, while the concept of inflation remains completely healthy.”

But some are cheerfully pulling down the curtain on inflation. Paul Steinhardt of Princeton University, who helped develop inflationary theory but is now a scathing critic of it, says that while the new study may be a blow for the theory, it pales in significance compared with inflation’s other problems. He says the idea that inflationary theory produces any observable predictions at all – even those potentially tested by BICEP2 – is based on a faulty simplification of the theory.

Because of quantum fluctuations, inflation is thought to produce an infinite multitude of universes that exhibit every conceivable property. “That means it doesn’t make any sense to say what inflation predicts, except to say it predicts everything,” he says. “If it’s physically possible, then it happens in the multiverse someplace.”

As an alternative, Steinhardt suggests the universe might have existed before the big bang, and slowly collapsed in a big crunch, before bouncing back and expanding anew – over and over. That could explain the universe’s smoothness without invoking multiverses. Not finding gravitational waves in the years to come will be the start of evidence for this theory, he says. Other observable predictions are being developed, but it’s a relatively new theory and more work is needed.

The next step is to see what can be gleaned from the Planck data – due in the next month – about the exact nature of cosmic dust. Whatever the result, with BICEP2 in place and several new instruments on the way, all the cosmologists New Scientist spoke to say it is an exciting time.

This article appeared in print under the headline “The rise and fall of cosmic inflation”